From: Sergey Bronnikov <sergeyb@tarantool.org>

Reported by Mathias Westerdahl.

(cherry picked from commit 633f265f67f322cbe2c5fd11d3e46d968ac220f7)

Lua 5.1 Reference Manual [1] defines a function `lua_concat`, that:

> void lua_concat (lua_State *L, int n);
>
> Concatenates the n values at the top of the stack, pops them, and leaves
> the result at the top.

Without the patch `lua_concat()` behaved incorrectly with userdata with
defined `__concat` metamethod. The problem is GC64-specific.

Assuming we have three literals and a userdata with defined "__concat"
metamethod on top of the Lua stack:

1 [string]
2 [string]
3 [string]
4 [string]
5 [userdata] <--- top

On attempt to concatenate *two* items on top of the Lua stack,
`lua_concat()` concatenates *four* items and leaves result on the top:

1 [string]
2 [string][string][string][userdata] <--- top

The problem is in incorrect calculation of `n` counter in the loop in
implementation of function `lua_concat`. Without the fix `n` is equal to
3 at the end of the first iteration and therefore it goes to the next
iteration of concatenation. In the fixed implementation of
`lua_concat()` `n` is equal to 1 at the end of the first loop iteration,
decremented in a loop postcondition and breaks the loop.

The patch fixes incorrect behaviour.

1. https://www.lua.org/manual/5.1/manual.html

Sergey Bronnikov:
* added the description and the test for the problem

Part of tarantool/tarantool#8825
---
PR: https://github.com/tarantool/tarantool/pull/9176
Branch: https://github.com/tarantool/luajit/commits/ligurio/lj-881-fix-concat
Issues:
- https://github.com/LuaJIT/LuaJIT/issues/881
- https://github.com/tarantool/tarantool/issues/8825

Changes:
- fixed typos
- updated assertions (bool asserts replaced with int assertions)
- replaced userdata with struct with userdata with integer per Max's request
that's all, folks

src/lj_api.c | 2 +-
.../lj-881-fix-lua-concat.test.c | 95 +++++++++++++++++++
2 files changed, 96 insertions(+), 1 deletion(-)
create mode 100644 test/tarantool-c-tests/lj-881-fix-lua-concat.test.c

diff --git a/src/lj_api.c b/src/lj_api.c
index 05e02029..3bacad33 100644
--- a/src/lj_api.c
+++ b/src/lj_api.c
@@ -801,7 +801,7 @@ LUA_API void lua_concat(lua_State *L, int n)
L->top -= n;
break;
       }
- n -= (int)(L->top - top);
+ n -= (int)(L->top - (top - 2*LJ_FR2));
       L->top = top+2;
       jit_secure_call(L, top, 1+1);
       L->top -= 1+LJ_FR2;
diff --git a/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
new file mode 100644
index 00000000..1b1f146f
--- /dev/null
+++ b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
@@ -0,0 +1,95 @@
+#include <string.h>
+
+#include "lua.h"
+#include "lauxlib.h"
+
+#include "test.h"
+#include "utils.h"
+
+/*
+ * This test demonstrates LuaJIT's incorrect behaviour, when
+ * calling `lua_concat()` with userdata with __concat metamethod.
+ * See https://github.com/LuaJIT/LuaJIT/issues/881 for details.
+ */
+
+#define MT_NAME "int"
+
+static int INFO_concat(lua_State *L)
+{
+ const char *s = luaL_checkstring(L, 1);
+ int *n = (int *)luaL_checkudata(L, 2, MT_NAME);
+ /* Do non-default concatenation. */
+ lua_pushfstring(L, "%s + %d", s, *n);
+ return 1;
+}
+
+static const luaL_Reg mt[] =
+{
+ {"__concat", INFO_concat},
+ {NULL, NULL}
+};
+
+static int lua_concat_testcase(void *test_state)
+{
+ /* Setup. */
+ lua_State *L = test_state;
+ const int top = 4;
+
+ /* Create metatable and put it to the Lua registry. */
+ luaL_newmetatable(L, MT_NAME);
+ /* Fill metatable. */
+ luaL_register(L, 0, mt);
+ lua_pop(L, 1);
+
+ assert_int_equal(lua_gettop(L), 0);
+
+ lua_pushliteral(L, "C");
+ lua_pushliteral(L, "B");
+ lua_pushliteral(L, "A");
+
+ int *n = (int *)lua_newuserdata(L, sizeof(int));
+ *n = 100;
+
+ luaL_getmetatable(L, MT_NAME);
+ lua_setmetatable(L, -2);
+
+ assert_int_equal(lua_gettop(L), top);
+
+ /* Test body. */
+
+ /*
+ * void lua_concat (lua_State *L, int n);
+ *
+ * Concatenates the n values at the top of the stack,
+ * pops them, and leaves the result at the top. If n is 1,
+ * the result is the single value on the stack; if n is 0,
+ * the result is the empty string [1].
+ *
+ * 1. https://www.lua.org/manual/5.1/manual.html
+ */
+
+ /* Concatenate two elements on top. */
+ lua_concat(L, 2);
+
+ const char *str = lua_tostring(L, -1);
+ assert_int_equal(lua_gettop(L), top - 2 + 1);
+ const char expected_str[] = "A + 100";
+ assert_str_equal(str, expected_str, strlen(expected_str));
+
+ /* Teardown. */
+ lua_settop(L, 0);
+
+ return TEST_EXIT_SUCCESS;
+}
+
+int main(void)
+{
+ lua_State *L = utils_lua_init();
+ const struct test_unit tgroup[] = {
+ test_unit_def(lua_concat_testcase),
+ };
+ const int test_result = test_run_group(tgroup, L);
+ utils_lua_close(L);
+
+ return test_result;
+}
--
2.34.1